Articles requiring high strength/weight ratio, such as aircraft structural parts, are commonly fabricated from polymerized foam material such as polymethacrylimide which has the commercial trade name Rohacell WF Rigid Foam. Such material is rigid at room temperature and is commercially available only in the form of orthogonal billets. At elevated temperature the material softens and may be reshaped by forming tools; however the thermal characteristics of this type of foam material makes it difficult to form into contoured parts because (a) being a good insulator of heat, it has a very narrow range of forming temperature, and (b) lack of heat retention causes the material temperature to drop rapidly below forming temperature when a workpiece is removed from the heating oven. For these reasons, prior to this invention, it has been necessary to heat both the forming tool and the foam workpiece to the forming temperature, and there was a tendency for material to creep from the weight of the tool; consequently forming was both costly and dimensionally inaccurate.
Prior art attempts at forming workpieces include the method detailed in U.S. Pat. No. 3,458,608 to Russell et al relating to a process of sequential partial expansions, compressing a shaped solid plastic workpiece, steeped in a gas-producing foam agent, between a flexible male membrane plunger and a rigid female mold.
U.S. Pat. No. 3,988,191 to Matsui shows methods and equipment for pressing foam thermoplastic workpieces between a rigid male plunger and a rigid female mold to form two halves, which are then welded together to form a vessel.
U.S. Pat. No. 3,948,710 to Harvey shows a method for forming a foam sandwich structure whose skins consist of paper layers, in a three-bar-roller forming mechanism; and, U.S. Pat. No. 3,041,660 to Fink shows a heated-die method for curing an elongated raw mass of thick liquid thermosetting resin which has been preformed into a rod-like shape and enveloped in a thin release film, to produce a solid rod.
Japanese Pat. No. 58-12730 discloses a method for putting a bend in a thin, flat sandwich structure by pressing it between a rigid male plug and a rigid female mold.
Unlike these prior art patents, the present invention utilizes a unique compliant female mold comprising an elastomeric belt stretched over a pair of variably-spaced sliding rollers adapted to dynamically and progressively conform to the shape of a rigid male plunger in the forming process, and encloses the foam workpiece in a heat-retaining envelope to allow forming in an unheated press. The present invention does not require application of paper skins, moistening of one skin or passing through a three-bar-roller forming mechanism, and does not produce wrinkles in the sandwich structure. Moreover, the present invention does not require the forming mechanism to supply heat to the workpiece.
According to the techniques of the invention, there is provided a method and apparatus for press forming an article of a required shape from a flat workpiece of structural foam material. According to the method, a flat workpiece of foam material is heated to a forming temperature and then supported by a compliant female mold member which is initially stretched flat. A male mold member is advanced against said heated workpiece and against the resistance of said compliant female mold member to form said workpiece to the desired shape. In a preferred form of the invention, the male mold member is held against said workpiece until the workpiece cools and hardens as the article of the desired shape.
According to the apparatus aspect of the invention, there is provided a forming press for shaping an article of required shape from a flat workpiece of foam including a rigid male forming punch defining the inside shape and dimensions of said article. A compliant female mold member initially stretched flat is provided to be disposed below the workpiece. Means are provided to lower the male mold member against the workpiece and against the resistance of the compliant female mold member so that the workpiece is progressively wrapped around said male mold member thus forming the workpiece to the required shape.
In a preferred form of the invention, the workpiece is first enclosed within a heat retaining envelope and heated to a forming temperature. The heat retaining envelope remains on the workpiece throughout the forming steps.
According to preferred techniques of the invention, the compliant female mold member is supported by roller means adapted to slide apart during the descending of the male mold member to compress the workpiece inwardly around said male mold member to retain the shape of such member and to prevent spring back action of the workpiece. In a further preferred form of the invention, elastic padding and adjustable tension springs are utilized to reduce crushing of the heated workpiece at high temperature and to maintain uniformed thickness of the workpiece.
It was an object of this invention to provide a method of forming heated foam which does not require the heating of the forming tools.
It was a further object of this invention to fabricate complicated shape of structural foam materials to correct dimensions.
It was a further object of this invention to produce shaped structural foam articles that had desired contours and were free of processing or tooling marks.
These and other objects of the invention will be apparent from the accompanying drawings and the description thereof which describe a preferred embodiment of the invention.